In recent years, as semiconductor integrated circuits have become finer, projection lithographic techniques using X-rays (which have a shorter wavelength (11 to 14 nm) than conventional ultraviolet light) instead of ultraviolet light have been developed in order to improve the resolving power of optical systems (which is limited by the diffraction limit of light) (for example, see D. Tichenor, et al., “SPIE,” 1995, Vol. 2437, p. 292). Such techniques have recently acquired the name of EUV (extreme ultraviolet) lithography, and have shown promise as techniques for obtaining a resolving power of 70 nm or finer, which cannot be realized with conventional photolithography using light rays with a wavelength of 190 nm.
The complex refractive index n of a substance in the X-ray wavelength region is expressed by n=1−δ−ik (δ and k are real numbers, and i is a symbol indicating complexity). The imaginary part k of this refractive index expresses X-ray absorption. Since δ and k are extremely small compared to 1, the refractive index in this region is extremely close to 1. Accordingly, conventional transmissive refraction type optical elements such as lenses cannot be used, and optical systems utilizing reflection are used instead. In the case of oblique-incidence optical systems that utilize total reflection to reflect X-rays that are incident on the reflective surface from an inclined direction, the reflectivity is extremely small at angles of incidence that are smaller than the critical angle θc of total reflection (approximately 20° or smaller at a wavelength of 10 nm) (i.e., angle of incidence that are close to perpendicular). Here, furthermore, the angle of incidence refers to the angle formed by the normal of the plane of incidence and the optical axis of the incident light.
Accordingly, multilayer film reflective mirrors are used in which numerous reflective surfaces (several tens to several hundreds of layers in one example) are formed by laminating a substance having an interfacial amplitude reflectivity that is as high as possible, and the thicknesses of the respective layers are adjusted on the basis of light interference theory so that the phases of the respective reflected waves match. These multilayer film reflective mirrors are formed by alternately laminating, on the surface of a substrate, substances in which the difference between the refractive index in the X-ray wavelength region used and the refractive index in a vacuum (=1) is large,. and substances in which this difference is small.
Furthermore, since these multilayer film reflective mirrors can also reflect X-rays that are perpendicularly incident, an optical system can be constructed which has a smaller aberration than an oblique-incidence optical system using total reflection.
Moreover, such multilayer film reflective mirrors have a wavelength dependence which is such that X-rays are strongly reflected in cases where Bragg equation2d sinθ=nλ(d: periodic length of multilayer film, θ: angle of incidence, λ: wavelength of X-rays) is satisfied. Therefore, various factors must be selected so that this equation is satisfied.
Known examples of multilayer films used in multilayer film reflective mirrors include films using a combination of W/C multilayer films in which tungsten (W) and carbon (C) are alternately laminated, Mo/C multilayer films in which molybdenum (Mo) and carbon are alternately laminated, and the like. Furthermore, these multilayer films are formed by thin film forming techniques such as sputtering, vacuum evaporation and CVD (chemical vapor deposition).
Moreover, if an Mo/Si multilayer film in which molybdenum (Mo) layers and silicon (Si) layers are alternately laminated is used in the wavelength region in the vicinity of 13.4 nm, a reflectivity of 67.5% can be obtained in the case of perpendicular incidence (angle of incidence: 0°), and if an Mo/Be multilayer film in which Mo layers and beryllium (Be) layers are alternately laminated is used in the wavelength region in the vicinity of 11.3 nm, a reflectivity of 70.2% can be obtained in the case of perpendicular incidence (for example, see C. Montcalm, “Proceedings of SPIE,” 1998, Vol. 3331, p. 42). Reflective mirrors using such multilayer films are also applied to reduction projection lithographic technology using soft X-rays that is referred to as EUVL (extreme ultraviolet lithography).
FIG. 3 is a sectional view showing in model form the structure of a multilayer film reflective mirror used in conventional EUVL. This multilayer film reflective mirror 41 is a mirror in which an Mo/Si multilayer film 45 is formed on a substrate 43. In this Mo/Si multilayer film 45, an Mo layer 47 and an Si layer 49 are taken as one layer pair, and approximately 40 to 50 such layer pairs are laminated. The periodic length of this Mo/Si multilayer film 45 (i.e., the thickness of one layer pair) is approximately 7 nm, and the ratio (Γ) of the thickness of one Mo layer to the periodic length is approximately 0.35 to 0.4. Furthermore, the surface (upper surface in the figure) of the substrate 43 ordinarily has a concave shape; however, in order to simplify the description, a portion of the multilayer film reflective mirror is made horizontal in the figure, and the number of laminated layers is abbreviated.
Incidentally, the multilayer film reflective mirror 41 is manufactured by sputtering (ion beam sputtering, magnetron sputtering, or the like), electron beam deposition, or the like; here, the high-reflectivity Mo/Si multilayer film 45 generally has a compressive internal stress of approximately −350 MPa to −450 MPa. As a result, the following problem arises: namely, the substrate 43 of the multilayer film reflective mirror 41 is caused to undergo deformation by the compressive internal stress of the Mo/Si multilayer film 45, so that wavefront aberration is generated in the optical system, thus causing a deterioration in the optical characteristics.
Accordingly, in order to reduce the compressive stress of a multilayer film with a high X-ray reflectivity, a technique has been reported in which a first multilayer film is formed on a substrate, and a multilayer film (second multilayer film) with a high X-ray reflectivity is formed on top of this first multilayer film, so that the stress of the multilayer film reflective mirror as a whole is reduced (for example, see E. Zoethout, et al., “SPIE Proceedings,” 2003, Vol. 5037, p. 872, and M. Shiraishi, et al., “SPIE Proceedings,” 2003, Vol. 5037, p, 249). Here, the periodic length of the first multilayer film is substantially the same as the periodic length of the second multilayer film, so that Γ is comparatively large (e.g., Γ=0.7). Since such a first multilayer film has a tensile stress, the compressive stress of the second multilayer film can be reduced.
Conventional stress reduction techniques will be described with reference to FIGS. 4 and 5.
FIG. 4 is a diagram showing the stress in a multilayer film with respect to Γ in a case where an Mo/Si multilayer film with a periodic length of 7.2 nm and a laminated layer number of 50 layer pairs was formed by sputtering with Γ varied. In FIG. 4, the horizontal axis expresses Γ (−), which is the ratio of the thickness of one Mo layer to the periodic length. Furthermore, Γ=0 indicates an Si single-layer film with a thickness of 250 nm, and Γ=1 indicates an Mo single-layer film with a thickness of 250 nm. Moreover, in FIG. 4, the vertical axis expresses the stress (MPa) of the film, with negative values indicating a compressive stress, and positive values indicating a tensile stress. The stress of the Mo/Si multilayer film varies according to Γ; it is seen that in the range in which Γ is smaller than approximately 0.5, the stress is a compressive stress, while in the range in which Γ is larger than approximately 0.5, the stress is a tensile stress. As was described above, since Γ of the second multilayer film which has a high reflectivity is approximately 0.35 to 0.4, this film has a compressive stress of approximately −350 MPa to −450 MPa. On the other hand, by using a multilayer film having a Γ value that is greater than approximately 0.5 as the first multilayer film, it is possible to generate a tensile stress in the first multilayer film. Accordingly, the internal stress of the multilayer film as whole can be reduced by combining a second multilayer film that has a compressive stress and a first multilayer film that has a tensile stress.
FIG. 5 is a sectional view which shows the structure of a conventional low-stress multilayer film reflective mirror in model form. In this multilayer film reflective mirror 51, a first multilayer film 57 is formed between a substrate 53 and a second multilayer film 55. The second multilayer film 55 is an Mo/Si multilayer film consisting of Mo layers 551 and Si layers 553; in this film, the periodic length is set at 7.2 nm, Γ is set at 0.35, and the number of laminated layers is set at 50 layer pairs, so that a high X-ray reflectivity can be obtained. On the other hand, the first multilayer film 57 is an Mo/Si multilayer film consisting of Mo layers 571 and Si layers 573, with the periodic length set at 7.2 nm, Γ set at 0.7, and the number of laminated layer set at 30 layer pairs. Furthermore, in order to simplify the description, a portion of the multilayer film reflective mirror is made horizontal in the figure, and the number of laminated layers is abbreviated. In this multilayer film reflective mirror 51, the second multilayer film 55 has a Γ value of 0.35, and therefore has a compressive stress, while the first multilayer film 57 has a Γ value of 0.7, and therefore has a tensile stress. Accordingly, the internal stress of the multilayer film as a whole can be reduced.
However, when multilayer film reflective mirrors are actually manufactured using such a conventional stress reduction technique, the following problem is encountered: namely, although the internal stress of the multilayer films is reduced, the X-ray reflectivity drops.